Our work carried out by support of the present PHS Research Grant has clarified that the response of the vertebrate photoreceptors, both rods and cones, is always hyperpolarizing irrespective of the mode, intensity and wavelength of the stimulating light, and that the hyperpolarizing response is accompanied by an increase in the membrane resistance. The ionic mechanism for such hyperpolarizing response was studied with the result that the sodium channels in the outer segment membrane remain open in the dark and are closed by light. The response of the vertebrate photoreceptors was thus shown to be just backward in its mechanism to the response of most receptors including invertebrate photoreceptors in which the sodium channels are opened by stimulation. The objectives of our research at this phase are to clarify the significance of the hyperpolarizing response of the vertebrate photoreceptors relative to the information transmission from the receptors to postsynaptic neurons (horizontal and bipolar cells) and to answer the question of when and what kind of transmitter(s) is released from the synaptic terminals of rods and cones. To accomplish these objectives, we intend to continue measuring the membrane resistance change accompanying the response to light of single horizontal and bipolar cells by means of intracellular micropipettes and a bridge circuit, and to study the effect of putative transmitter substances and of Ca ions-free external media upon the membrane potential and the membrane resistance of these postsynaptic neurons.